Fix issue #15, add attribute to avoid a warning by GCC

[0x3333]

As mentioned in the issue #15, added an attribute to avoid GCC issue a warning regarding a fall through.

[devanlai]

Hrm, I thought the existing /* Fall through */ would be sufficient, but apparently gcc is very picky about the location of the fall-through comment.

Unfortunately, adding the __attribute__ ((fallthrough)) generates a warning: empty declaration. However, it still compiles and all else equal, I'm fine with having warnings on ancient compiler versions if it makes the latest compiler version warning-free.